The present invention relates to the purification of gaseous inorganic halides.
Gaseous inorganic halides have widespread commercial applications. Such compounds have been known in the chemical arts for many years. These compounds are widely used as catalysts in polymers, pharmaceuticals, fine chemical synthesis and in the manufacture of electronic and semiconductor devices. Boron trifluoride, in particular, is used in the electronics industry for ion implantation and as a p-type dopant for semiconductor devices. Due to the purity requirements in many of the above applications, particularly in the electronics and semiconductor fields, there is an ever increasing need for ultra high purity gaseous inorganic halide compounds.
Most gaseous inorganic halide compounds are highly reactive to air and water and therefore must be handled in air-free, inert atmospheres. The manufacturing and purification of these compounds can be extremely difficult due to this high reactivity. A particular gaseous inorganic halide of commercial interest is boron trifluoride. Boron trifluoride is a strong Lewis acid that complexes with water, polar organic solvents, and other compounds having at least one unshared pair of electrons. The synthesis of boron trifluoride has been disclosed in U.S. Pat. Nos. 2,148,514 and 2,196,907. Boron trifluoride is a colorless gas which forms dense white fumes in moist air. It has a boiling point of xe2x88x92100xc2x0 C., a melting point of xe2x88x92127xc2x0 C. and like many inorganic halides it is extremely corrosive. Commercially available boron trifluoride typically has a purity of 99.7%. The major impurities are air, at 4,000 to 1,700 parts per million (PPM) and silicon halide (as silicon tetrafluoride) at 300 to 50 PPM. Other impurities present as artifacts of the manufacturing process include sulfate (10 PPM), hydrogen fluoride (25 PPM), hydrogen chloride (10 PPM), sulfur dioxide and boron trifluoride dihydrate are found in trace amounts. While these amounts are typical, the purity profile of any single lot of gaseous inorganic halide, particularly boron trifluoride, may vary.
Generally, boron trifluoride is purified through either distillation or adsorption on zeolites to remove some contaminants. Using these techniques, atmospheric gases such as nitrogen, oxygen and carbon dioxide can be reduced to concentrations of 10-20 PPM. However, other impurities listed above still remain. Therefore, a purification means is needed to remove most of the remaining impurities from gaseous inorganic halides and in particular boron trifluoride so that the concentration of the total impurities is reduced to less than about 20 PPM, a concentration suitable for use in electronic and semiconductor applications. It should be noted however that the purified gas provided by the present invention may be used in any application where high purity gas is necessary.
The present invention provides an improved purification method for gaseous inorganic halides and in particular boron trifluoride. Preferred processes according to the present invention can meet the needs and demands of today""s electronic and semiconductor needs. Preferred processes according to the present invention specifically provides a method of reducing the total contaminant concentration to less than 20 PPM. The present invention also provides an apparatus for practicing the purification method.
The present invention generally provides an apparatus and a method of purifying gaseous inorganic halide compounds. Specifically, the present invention involves a process whereby the crude, or unpurified gaseous inorganic halide is contacted with a material capable of reacting and/or adsorbing impurities thereby reducing the same.
One aspect of this invention provides a process to reduce or eliminate impurities in gaseous inorganic halides to concentrations acceptable for use in high purity applications such as electronic and semiconductor devices. This purification is achieved by contacting the gaseous inorganic halide with a reactive substance. In a preferred embodiment of the invention, the reactive substance is a metal. The reactive metal is finely divided or otherwise provided in a form having a high surface area such as shot, foil, sheets, granules or powders. The reactive metal may also be plated or deposited onto or into an inert support. In another preferred embodiment, the reactive metal is packed in a column such that the crude gas is allowed to pass through and contact the metal. In yet another preferred embodiment of the present invention, the metal is provided in a powdered or pelletized form and contained in a fluidized bed such that the crude gaseous inorganic halide material is allowed to pass through thereby contacting the metal and thus react and/or adsorb impurities.
In another aspect of the present invention, the reactive metal used is preferably an alkaline metal, alkaline earth metal or an alloy of such metal. In a preferred embodiment of the present invention, the reactive metal selected is lithium or calcium. In yet another preferred embodiment of the present invention, the reactive metal selected may be an alloy of lithium or calcium combined with Group III or Group IV elements, and preferably one or more of the following elements: silicon, aluminum, germanium, boron and calcium. In another preferred embodiment of the invention, the gaseous inorganic halide is contacted with a reactive metal at a temperature above room temperature. (Room temperature is defined as any ambient temperature between about 20xc2x0 C. to about 28xc2x0 C.) In another preferred embodiment of the invention, the reactive metal is heated to a temperature below that of its melting point. In another preferred embodiment of the present invention, the contact time, or residence time, of the gas on the reactive metal is preferably between 30 seconds and 30 minutes, depending on other variables such as the reactor size and shape, the reactive metal used, the inorganic halide used and its purity.
In another aspect of the present invention, a method is provided whereby the crude gaseous inorganic halide is purified by contact with, or distillation over, an adsorbent material prior to being contacted with the reactive metal. In a preferred embodiment of the present invention, the adsorbent material can be molecular sieves. In an even more preferred embodiment of the present invention, 5A molecular sieves are used to contact the crude gaseous inorganic halide prior to contacting the reactive metal.
In another aspect of the present invention, the crude gaseous inorganic halide is distilled. In one preferred embodiment of the present invention, the inorganic halide may be distilled in the absence of any adsorbant or reactive substance. In another preferred embodiment of the present invention, the distillation of the crude inorganic halide is performed prior to, during or after it is contacted with either the molecular sieves or the reactive metal. In a preferred embodiment of the present invention, the distillation step preferably includes cryogenically condensing the gaseous inorganic halide at a temperature at or below its melting point followed by evacuation or pumping off the uncondensed gases. In yet another aspect of the present invention, the gaseous inorganic halide purified may be BF3, BCl3 SiF4, GeF4, PF3, PF5, AsF3, AsF5, SbF3, SbF5 and mixtures thereof.
In another aspect of the present invention, an apparatus is provided allowing for the gaseous inorganic halide to be purified in an inert, anhydrous environment.